1. Field of the Invention
This invention relates to an image heating apparatus applied to an image forming apparatus such as a copier a printer or the like, and particularly to a apparatus for detecting the rotational speed of a rotary member.
2. Related Background Art
Heretofore, in an image forming apparatus such as a copier, LBP, a facsimile apparatus, a microfilm reader printer, an image display apparatus or a recording machine, as a heating apparatus (an image heating apparatus or an image heating and fixing apparatus) for heating and fixing on the surface of a recording material (such as a transfer material sheet, an electrofax sheet, an electrostatic recording sheet or printing paper) an unfixed toner image corresponding to desired image information formed and borne on the surface of the recording material by an indirect (transfer) method or a direct method by suitable image process means such as electrophotography, electrostatic recording or magnetic recording as a permanent fixed image by the use of a bisualizing agent (toner) comprising heating meltable resin or the like, use has often been made of a heat roller system for heating the recording material as a material to be heated by a heating roller as a heating member maintained at a predetermined temperature and a pressing roller as a pressing member having an elastic layer and urged against the heating roller while pinching and conveying the recording material therebetween.
Besides this, various systems such as a flash heating system, an open heating system and a heat plate heating system are known and practically used.
Recently, instead of these systems, there has been devised a heating apparatus (film heating system) of a type which has a fixed and supported heating member, heat-resisting film (fixing film) conveyed while being urged against the heating member, and a pressing member for bringing a recording material as a material to be heated into close contact with the heating member, and in which the heat of the heating member is imparted to the recording material through the film to thereby heat and fix an unfixed image formed and borne on the surface of the recording material.
This apparatus can be widely used not only as a fixing apparatus but also as an apparatus for heating a recording material bearing an image thereon and changing the surface property (such as gloss) thereof, and means heating a member to be heated such as tentatively fixing an image thereon.
The heating apparatus of such a film heating type can use a heating member of low heat capacity quick in temperature rise and thin film and therefore, has advantages that the saving of electric power and the shortening of wait time (quick starting property) becomes possible, and that the temperature rise in the interior of a main apparatus such as an image forming apparatus can be lowered, and Is thus effective.
In an apparatus like the heating apparatus of the heat roller type or the film heating type in which a material to be heated is introduced into the pressure contact nip portion between a heating member and a pressing member and the material to be heated is heated while being pinched and conveyed by the two members, a fluctuation occurs to the pinching and conveyance speed of the material to be heated depending on the temperature state of constituent members. For example, in a heating apparatus of the film heating type and of a pressing member driving type in which a pressing member (hereinafter referred to as the pressing roller) urged against a heating member with film interposed therebetween is rotatively driven, whereby the film or the film and a material to be heated together are pinched and conveyed in the pressure contact nip portion between the heating member and the pressing roller while the film is slidingly moved relative to the heating member, the temperature of the pressing roller rises with the operation of the apparatus, whereby the outer diameter of the pressing roller is increased by the thermal expansion of the rubber portion thereof.
The pressing roller is usually rotatively driven at a constant number of rotations and therefore, when the pressing roller is at a high temperature, the thermal expansion thereof becomes greater and the rotational peripheral speed thereof is increased and the pinching and conveying speed of the material to be heated becomes higher than when the pressing roller is at a low temperature.
That is, depending on the temperature state of the pressing roller, a difference occurs to the pinching and conveying speed of the material to be heated by the heating apparatus.
Therefore, for example, when this heating apparatus is used as an image heating and fixing apparatus in an image forming apparatus, the conveyance of a recording material at an image making portion which is a processing portion upstream of the heating apparatus, for example, an image transfer portion, is kept constant and therefore, when the recording material comes from the image transfer portion to the recording material pressure contact nip portion (fixing portion) of the heating apparatus and assumes its pinched and conveyed state, there is created a state in which the pinching and conveying speed of the recording material at the fixing portion of the heating apparatus is greater than the conveying speed of the recording material at the image transfer portion when the pressing roller is in a high temperature state, and thus the heating apparatus pulls the recording material and under this influence, image blur occurs in the image transfer portion.
When in expectation of this recording material pulling phenomenon, the pinching and conveying speed of the recording material by the heating apparatus is initially set to a level lower than the conveying speed of the recording material at the image transfer portion, an unnecessary loop (looseness) is formed in the recording material at the recording material conveying portion between the image transfer portion and the fixing portion of the heating apparatus at a point of time whereat the temperature of the pressing roller is still low, and the direction of separation of the recording material from an image bearing member after the image transfer at the image transfer portion and the angle of entrance of the recording material into the fixing portion of the heating apparatus become unstable and therefore, the image scattering during the separation of the recording material from the image bearing member and the offset or the like at the fixing portion of the heating apparatus occur. Also, when the recording material used is thick, image blur occurs at the image transfer portion due to the rigidity of the recording material.
To eliminate the evils by the recording material pulling phenomenon due to the fluctuation in the recording material pinching and conveying speed of such a heating apparatus and the unnecessary loop forming phenomenon, it would occur to mind to lengthen the distance of the recording material conveying portion between the image transfer portion and the fixing portion of the heating apparatus and shorten the time for which the recording material is present at the image transfer portion and the fixing portion of the heating apparatus at a time.
However, in an image forming apparatus like an A3 machine using a long recording material, the recording material conveying distance between the image transfer portion and the fixing portion becomes too long, and the main body of the image forming apparatus has become considerably bulky.
Also, Japanese Patent Application Laid-Open No. 7-261584 proposes a method of detecting the amount of loop of a recording material or detecting the conveying speed of the recording material and varying the driving speed of a pressing roller. In this method, however, control cannot be done until the first recording material arrives at a heating apparatus (a fixing device), and when the recording material is to be conveyed at a high speed, control may not be in time.
There is also proposed a method of measuring the temperature of a pressing roller, and foreseeing the amount of expansion thereof and varying the driving speed, but in this method, the error is great and therefore more accurate control cannot be done.
Also, Japanese Patent Application Laid-Open No. 8-190298 proposes a method of providing a plurality of reflecting plates on film, reading the cycle of ON-OFF of a reflection type sensor to thereby detect the peripheral speed of the film, and sequentially varying the driving speed of a pressing roller so that the peripheral speed may become constant.
However, there has been desired a method more improved in the strength of adhesion of the reflecting plates to the film, the reflecting performance of the reflecting plates and the prevention of the staining of the reflecting plates.
It is an object of the present invention to provide an image heating apparatus which improves the strength of adhesion of a marking portion to a rotary member, the reflecting performance of the marking portion, and the prevention of the staining of the marking portion.
It is another object of the present invention to provide an image heating apparatus which comprises a rotary member contacting with a recording material bearing an image thereon, a marking portion provided on the rotary member, and detecting means for detecting the rotational speed of the rotary member by reflected light from the marking portion, and in which the image on the recording material is heated by heat from the rotary member side, and the marking portion has a mixture of an adhesive agent and a reflective material.
It is still another object of the present invention to provide an image heating apparatus which comprises a rotary member contacting with a recording material bearing an image thereon, a marking portion provided on the rotary member, and detecting means for detecting the rotational speed of the rotary member by reflected light from the marking portion, and in which the image on the recording material is heated by heat from the rotary member side, and the marking portion has a spherical reflecting member.
It is yet still another object of the present invention to provide an image heating apparatus which comprises a rotary member contacting with a recording material bearing an image thereon, a marking portion provided on the rotary member, and detecting means for detecting the rotational speed of the rotary member by reflected light from the marking portion, and in which the image on the recording material is heated by heat from the rotary member side, and the marking portion has a metallic member.
It is a further object of the present invention to provide an image heating apparatus which comprises a rotary member contacting with a recording material bearing an image thereon, a marking portion provided on the rotary member, and detecting means for detecting the rotational speed of the rotary member by reflected light from the marking portion, and in which the image on the recording material is heated by heat from the rotary member side, and the marking portion has a reflecting member, which is directly evaporated on the rotary member.
It is still a further object of the present invention to provide an image heating apparatus which comprises a rotary member contacting with a recording material bearing an image thereon, a marking portion provided on the rotary member, and detecting means for detecting the rotational speed of the rotary member by reflected light from the marking portion, and in which the rotary member has a base layer and a surface parting layer, the image on the recording material is heated by heat from the rotary member side, and the marking portion is a portion on which the surface parting layer is partly not provided and the base layer is exposed.
It is yet still a further object of the present invention to provide an image heating apparatus which comprises a rotary member contacting with a recording material bearing an image thereon, a marking portion provided on the rotary member, and detecting means for detecting the rotational speed of the rotary member by reflected light from the marking portion, and in which the image on the recording material is heated by heat from the rotary member side, the rotary member has a slit, and the marking portion has a reflecting member, which is wound through the slit.
It is another object of the present invention to provide an image heating apparatus which comprises a rotary member contacting with a recording material bearing an image thereon, a marking portion provided on the rotary member, and detecting means for detecting the rotational speed of the rotary member by reflected light from the marking portion, and in which the image on the recording material is heated by heat from the rotary member side, and the marking portion is a hole (aperture) having a reflecting member on the back side thereof.
It is still another object of the present invention to provide an image heating apparatus which comprises a rotary member contacting with a recording material bearing an image thereon, and a marking portion provided on the rotary member to detect the rotational speed of the rotary member, and in which the image on the recording material is heated by heat from the rotary member side, and the marking portion differs in surface resistance, dielectric constant or magnetism from other portions with respect to the circumferential direction of the rotary member.
It is yet still another object of the present invention to provide an image heating apparatus which comprises a rotary member contacting with a recording material bearing an image thereon, a marking portion provided on the rotary member, and detecting means for detecting the rotational speed of the rotary member by reflected light from the marking portion, and in which the image on the recording material is heated by heat from the rotary member side, the rotary member has a base layer, a surface parting layer and a primer layer provided between the base layer and the surface parting layer, and the marking portion has a mixture of the material of the surface parting layer and a reflective material.
Further objects of the present invention will become apparent from the following detailed description.